This invention relates to a microwave oven having a controllable frequency microwave power source, and more particularly to a microwave oven in which the oscillation frequency of its microwave power source is controlled depending on the load to be heated.
One of the main attractions of modern microwave ovens is that they can provide automatic heating. When an automatic heating system is employed, the level of output power of the microwave power source is controlled in a time division mode depending on the load to be heated. In a domestic or home-use microwave oven, a magnetron is employed as the microwave power source, and the microwave power generated from the magnetron is provided to the oven cavity to heat a load placed in the oven cavity to be heated with the microwave power. It is acknowledged that, in the microwave power generated from the magnetron, the proportion of the microwave power contributing to the heating of a load placed in the oven cavity (which proportion of power will be referred to hereinafter as available power) varies depending on the kind and amount of the load. Generally, the smaller the size of the load, the less the available power.
This is mainly due to a poor impedance match between the magnetron and the loaded oven cavity. How the heating efficiency of the modern microwave oven comprising an advanced automatic heating system can be maintained high for all types of loads to be heated is a technical problem to be solved from, among others, the viewpoint of energy saving.
In order that the microwave oven can operate with high heating efficiency, it is required to maintain a satisfactory impedance match between the loaded oven cavity and the microwave power source providing microwave power to this oven cavity.
Measures for maintaining a satisfactory impedance match between the loaded oven cavity and the microwave power source are classified into those in which one is to make variable the mechanism of the microwave transmission system and the other is to make variable the oscillation frequency of the microwave power source. U.S. Pat. No. 3,104,304 to Sawada employs the former measures and attempts to improve the heating efficiency by manipulating the electric field patterns in the oven cavity by changing the physical dimensions of the oven cavity.
The problem involved in this U.S. patent is the limitation placed on the load to be heated in order to maintain high efficiency. Further, to manipulate the electric field patterns in the cavity is not always effective in ensuring high efficiency.
U.S. Pat. No. 4,196,332 to MacKay B et al employs the latter measures and attempts to improve the efficiency by controlling the oscillation frequency of the microwave power source on the basis of the levels of reflected power from the oven cavity thereby maintaining a satisfactory impedance match between the microwave power source and the loaded oven cavity. A microwave oven having a controllable frequency microwave power source can maintain high efficiency for any load to be heated. However, the multimode cavity has the defects that the electromagnetic modes in the loaded cavity change as the load is being heated and/or that the initial resonant frequencies generating the electromagnetic modes in the loaded cavity shift to other frequencies as the load is being heated. The frequency generating the electromagnetic mode in the loaded cavity is generally correlated to the frequency reducing the reflected power from the loaded cavity. According to this description, in this cited microwave oven having a multimode cavity for receiving a load to be heated, to operate the microwave power source at frequencies at which the initial reflected power levels from the loaded cavity are below the predetermined reflected power level, reduces the efficiency for a special load as the load is being heated.
It is acknowledged that the selection of electromagnetic modes, i.e., the selection of electric field patterns or distributions in the oven cavity is an important factor for attaining uniform heating of a load to be heated. The selection of the electric field patterns is equivalent to the selection of the dimensions of the width, height and depth of the oven cavity. However, even when an oven cavity is so determined, all of a plurality of electric field patterns, i.e., electromagnetic modes established in the oven cavity cannot always contribute to the attainment of uniform heating of the load. Further, even when the electromagnetic mode suitable for attaining uniform heating of the load may be selected, it is impossible, as a matter of fact, to select the mode in accordance with the amount of reflected power detected from the multimode oven cavity. The information available as a result of the detection of the amount of reflected power teaches only that some electromagnetic modes are present in the oven cavity although the details of the electric field patterns are unknown. In the invention of MacKay B et al, the load is heated with microwave power at a plurality of frequencies generating different electric field patterns so as to attain uniform heating of the load, in an attempt to obviate the difficulty pointed out above. However, the frequencies are determined on the basis of the detector signal representative of the amount of reflected power in the initial condition of heating of the load. Therefore, in the case of a load whose physical properties tend to change with the progress of heating, the impedance match between the microwave power source and the loaded oven cavity will not always be maintained in a satisfactory state throughout the duration of heating.